This study used monoclonal antibodies to sheep MHC class II molecules as well as an L cell transfectant (T8.1) which expresses DRA and DRB genes to show that two distinct DR beta chains are expressed in the sheep. Two anti-beta chain specific monoclonal antibodies VPM37 and VPM43 react with DR antigen but not DQ antigen by ELISA. These two antibodies do not react with the DR beta chain expressed in the T8.1 cell line. Two-dimensional immunoblotting shows that these antibodies recognize a subgroup of the spots recognized by the DR-specific monoclonal antibody VPM57 which does react with the T8.1 beta chain. Amino-terminal sequence analysis of the alpha chain associated with VPM37 beta chain shows that this alpha chain is homologous to the human DR alpha chain strongly indicating that the beta chain is DR-like. VPM37 and VPM43 are shown to be directed against different epitopes on sheep MHC class II molecules so it is highly unlikely that the data can be explained by the presence of post-translational modifications or the existence of a very common allele. These data provide clear evidence for the expression of two distinct DR beta chains in the sheep.